(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor devices, and more specifically to a method used to create a gate structure, for a dynamic random access memory, (DRAM), device
(2) Description of Prior Art
The semiconductor industry is continually striving to improve device performance. The use of sub-micron features, for semiconductor devices, has allowed performance degrading, junction capacitances, to be decreased, resulting in a portion of the performance objectives to be realized. In addition the use of narrow channel lengths have also contributed to the objective of enhanced performance. The ability to form metal oxide semiconductor field effect transistor, (MOSFET), devices, with narrow channel lengths, is directly related to the width of an overlying gate structure. The channel length of the MOSFET device is defined by the self-alignment of a source/drain region, formed using the overlying gate structure as a mask. The gate structure, either a polysilicon, or a polycide, (metal silicide--polysilicon), gate structure, is created via an anisotropic reactive ion etching, (RIE), procedure, applied to a polysilicon, or polycide layer, using an overlying photoresist shape as an etch mask. For many applications, such as for DRAM devices, silicon nitride capped gate structures, are used, allowing subsequent procedures, such as the formation of self-aligned contact openings, to be made. The use of silicon nitride capped, gate structures, now demand that the anisotropic RIE procedure, define the desired gate structure pattern in a silicon nitride layer, as well as in the underlying polysilicon, or polycide layer. However the silicon nitride layer presents reflectivity problems during the photolithographic exposure procedure, used to create the photoresist shape, that is needed as the etch mask for definition of the sub-micron width, gate structure. The reflectivity from the silicon nitride layer results in unwanted exposure of portions of the photoresist layer, creating a photoresist shape, exhibiting a thinned, or necked, profile. When the photoresist shape, featuring a necked profile, is used as an etch mask, the resulting gate structure can have a defective, or unwanted profile, thus adversely influencing the underlying channel length, and subsequent device performance.
A method used to minimize the reflectance, encountered during the photolithographic exposure procedure, used to create the gate structure, photoresist shape, is the use of a bottom anti-reflective coating, (BARC), layer, placed between the overlying photoresist layer, and the underlying silicon nitride capping layer. A BARC layer, such as a silicon oxynitride, (SiON), layer however, if applied at a specific thickness range, does not provide the needed reflectance protection, for wide ranges of silicon nitride thicknesses. For example a SiON, BARC layer, at a thickness between about 100 to 300 Angstroms, will only supply the desired reflectance protection for very specific ranges of silicon nitride thicknesses. Therefore slight thickness variations in the silicon nitride capping layer, that can be encountered within a wafer, or from wafer to wafer, can result in unwanted profiles of the photoresist shape, used as a mask for gate structure definition, thus resulting in uncertain channel lengths.
This invention will describe a process that allows the reflectivity, presented by all thickness of silicon nitride, as a result of the photolithographic exposure procedure, to be minimized, or eliminated. This is accomplished via the use of a SiON BARC layer, with a thickness between about 820 to 920 Angstroms, providing the unexpected result of between one to two orders of magnitude, decrease in reflectivity, for a range of thicknesses of silicon nitride capping layers, between 200 to 3000 Angstroms. Prior art, such as Tsukamoto et al, in U.S. Pat. No. 5,600,165, describe a anti-reflective coating, however that prior art does not use the anti-reflective coating, as a bottom coating, for the definition of a MOSFET gate structure.